Color screening can be utilized for colonies grown in the laboratory to distinguish different types of colonies. For example, a blue/white (binary) color screen exploits a visible color difference between blue colonies and white colonies to identify colonies of interest for isolation and further study or processing.
The blue/white screen is typically conducted as an in situ beta-galactosidase assay. A gene encoding full-length beta-galactosidase (or a complementing portion thereof) may be introduced into a beta-galactosidase-deficient microorganism. Colonies of the microorganism expressing the gene have beta-galactosidase activity, whereas colonies that do not express the gene do not have the activity. These two types of colonies can be distinguished by growing the colonies in the presence X-gal, a substrate for beta-galactosidase. Hydrolysis of X-gal by beta-galactosidase converts the colorless substrate to a blue dye, which turns expressing colonies blue. In contrast, non-expressing colonies do not accumulate the blue dye and remain white.
Colony analysis systems are available for detecting images of colonies. However, these systems often are equipped with a grayscale (“black-and-white”) image detector. The grayscale image detector obtains an image by measuring of an amount of light incident on each pixel without recording the wavelength or color of that light. As a result, colony types of different color may not be reliably distinguishable in the image. Colonies of at least one type may be missed partially or completely or may be incorrectly identified as belonging to another type.
Better colony analysis systems are needed for identifying colonies of different color based on images obtained by a grayscale image detector.